This invention relates generally to optical systems. More specifically, it relates to a novel class of optical spectral power monitors in which polarization-sensitive effects are mitigated and the insertion loss is minimized. The optical spectral power monitors of the present invention are particularly suited for DWDM optical networking applications.
Dense wavelength division multiplexing (WDM) has become prevalent in optical communication networks, in response to high bandwidth (or capacity) demand. Along with the deployment of DWDM technology comes a need for a new generation of optical components and subsystems, including optical spectral (or channel) power monitors. A particularly desirable feature for these new optical spectral power monitors is the ability to resolve multiple spectral channels that occupy a broad spectrum range (e.g., C- or L-band) with increasingly narrower frequency spacing (e.g., 50 or 25 GHz). These optical spectral power monitors are also desired to be fast in response time, robust in performance, and cost-effective in construction.
Conventional spectral power monitors typically make use of an architecture in which a diffraction grating separates a multi-wavelength optical signal by wavelength into a spatial array of spectral channels, impinging onto an array of optical power sensors. By detecting the electrical signals thus produced by the optical power sensors, an optical power spectrum of the multi-wavelength optical signal can be derived. In order to provide enhanced spectral resolution in such a system, a diffraction grating with sufficient dispersion capability is required. High-dispersion diffraction gratings commonly known in the art (e.g., holographic gratings), however, are characteristically polarization-sensitive, rendering them unsuitable for the optical spectral power monitors employing the aforementioned architecture.
In view of the foregoing, it would be an advance in the art to overcome the prior limitations and provide a new type of optical spectral power monitors with enhanced spectral resolution in a simple and cost-effective construction.
The present invention provides a method and apparatus for spectral power monitoring by use of a polarization diversity scheme. The optical spectral power monitoring apparatus of the present invention comprises an input port for a multi-wavelength optical signal; a polarization-separating element that decomposes the multi-wavelength optical signal into first and second polarization components; a polarization-rotating element that rotates the polarization of the second polarization component by 90-degrees; a wavelength-disperser that separates the first and second polarization components by wavelength respectively into first and second sets of optical beams; and an array of optical power sensors (termed xe2x80x9coptical-sensing arrayxe2x80x9d herein) positioned to receive the first and second sets of optical beams.
In an exemplary embodiment of the present invention, the input port may be a fiber collimator, and the wavelength disperser may be a diffraction grating. In the event that the diffraction grating may provide higher diffraction efficiency for a p (or TM) polarization component than for an s (or TE) polarization component, the aforementioned first and second polarization components may correspond to the p-polarization and s-polarization components of the input multi-wavelength optical signal, respectively.
In situations where the first and second optical beams associated with the same wavelength are desired to impinge at substantially the same location (or within the same optical power sensor) on the optical-sensing array, an auxiliary polarization-rotating element may be implemented such that the first and second sets of optical beams are polarized in two orthogonal directions upon impinging onto the optical-sensing array. This eliminates any intensity interference fringes that may arise from the spatial overlap of the optical beams. The auxiliary polarization-rotating element may be disposed between the wavelength-disperser and the optical-sensing array, such that either of the first and second sets of optical beams undergoes a 90-degree rotation in polarization prior to impinging onto the optical-sensing array.
Alternatively, a modulation assembly may be utilized in the present invention to modulate the first and second sets of optical beams prior to impinging onto the optical-sensing array. The first and second sets of optical beams may be modulated to arrive at the optical-sensing array in a time-division-multiplexed sequence. The first and second sets of optical beams may alternatively be modulated in a frequency-division-multiplexed fashion, such that the first and second sets of optical beams impinging onto the optical-sensing array carry distinct xe2x80x9cditherxe2x80x9d modulation signals. In either case, the use of such a modulation assembly enables the first and second sets of optical beams to be separately detected, whereby an optical power spectrum (optical power level as a function of wavelength) associated with each orthogonal polarization component in the input multi-wavelength optical signal can be independently derived. The modulation assembly may be disposed along the optical path between the polarization-separating element along with the polarization-rotating element and the wavelength-disperser, thereby controlling the first and second polarization components. It may alternatively be implemented between the wavelength-disperser and the optical-sensing array, so as to control the first and second sets of optical beams.
The modulation assembly may comprise liquid crystal shutter elements, MEMS (micro-electro-mechanical-systems) shutter elements, or electro-optic intensity modulating elements known in the art. The modulating assembly may also be provided by an optical beam-chopper (e.g., a rotating disk equipped with at least one aperture), configured to introduce distinct modulations in two incident optical signals.
In the present invention, the polarization-separating element may be a polarizing beam splitter, a birefringent beam displacer, or other types of polarization-separating means known in the art. The polarization-rotating element (or the auxiliary polarization-rotating element) may be a half-wave plate, a liquid crystal rotator, a Faraday rotator, or any other suitable polarization-rotating means known in the art. The optical-sensing array may be provided by a photodiode array, or other types of optical power sensing elements known in the art. The wavelength-disperser may generally be a ruled diffraction grating, a holographic diffraction grating, a curved diffraction grating, an echelle grating, a transmission grating, a dispersing prism, or other types of wavelength-separating means known in the art. The input port may be a fiber collimator, coupled to an input optical fiber (e.g., a single mode fiber) transmitting the multi-wavelength optical signal.
The employment of the aforementioned polarization diversity scheme enables the optical spectral power monitoring apparatus of the present invention to minimize the insertion loss, while providing enhanced spectral resolution in a simple and cost-effective construction (e.g., by advantageously making use of high-dispersion diffraction gratings commonly available in the art). Further, by introducing distinct modulations to the first and second sets of optical beams prior to impinging onto the optical-sensing array, an optical power spectrum associated with each polarization component in the input multi-wavelength optical signal can be separately determined, which might be desirable in some applications.
As such, the present invention provides a new type of optical spectral power monitors with enhanced spectral resolution and minimized insertion loss, that can be utilized in a variety of applications including DWDM optical networking applications.
The novel features of this invention, as well as the invention itself, will be best understood from the following drawings and detailed description.